The typical vocalization of many primates when separated from conspecifics is a tonal sound of relatively long duration and little abrupt frequency change or noise. These structural features characterize the isolation calls of primates as phylogenetically diverse as lemurs, macaques, and human infants, suggesting that the same genetic program determining isolation call structure may be widespread across the primate order. This same basic structure is also found in the isolation calls of squirrel monkeys (Saimiri). Analysis of the squirrel monkey isolation call has revealed a subtle but consistent difference in structural details of isolation calls from two physically distinct squirrel monkey types or species, the Gothic-arch and Roman-arch types. Studies leading up to the present project showed that these structural differences are present in newborn infants and persist regardless of subsequent experience, suggesting a major role for genetic determination of the isolation call's species-specific attributes. In the present project, adults of both Saimiri types are cross-bred and the isolation calls of the hybrid offspring analyzed for resemblance to parental phenotype. Roman-arch adults from Peru bred with Gothic-arch adults from Peru or Colombia produce offspring whose isolation calls resemble the Roman-arch parent. In the majority of cases the degree of resemblance is only approximate; discriminant analysis indicates that measures of hybrid isolation call structure typically produce discriminant scores intermediate to those of the parents, or to isolation calls of age-matched pure-bred offspring reared under the same conditions. In related crossbreeding studies, Roman-arch males from Bolivia were mated with females from Guyana, representing the most geographically and phenotypically disparate South American populations of Saimiri. Hybrid offspring from these pairings produce isolation calls closely resembling the Gothic-arch (maternal) phenotype, while their physical appearance more nearly resembles that of the Roman-arch parent.